1. Field of the Invention
The present invention relates to a high tensile strength hot-rolled steel sheet to be used in large electric equipment such as a rotor of generator, particularly to a high tensile strength hot-rolled steel sheet having excellent workability, high magnetic permeability and high magnetic flux density in a strong magnetic field, and strength of at least 490 MPa, and relates to a method for manufacturing thereof.
2.Description of Related Arts
For improving the performance of large electric equipment such as a rotor of generator, the steel sheet to be used therein is requested, in recent years, to have, adding to mechanical characteristics, high magnetic permeability and high magnetic flux density in a strong magnetic field, for example, 2.00 Tesla (T) or higher magnetic flux density B300 in a magnetic field of 30 kA/m. Since the magnetic permeability of a steel sheet increases with a decrease in precipitates and with an increase in crystal grain size, and since the magnetic flux density thereof increases with a decrease in amount of non-magnetic elements, a high tensile strength hot-rolled steel sheet based on a very low carbon steel added with large amount of solid solution strengthening elements Si, Al, and P has been used for the use of the large electric equipment.
The addition of these solid solution strengthening elements, however, cannot easily give high tensile strength of 490 MPa or more, and significantly degrades workability, further reduces magnetic flux density in a strong magnetic field. Accordingly, if the balance between strength, workability, and magnetic characteristics is considered, it is preferable to increase the tensile strength by refining ferritic grains and by forming fine precipitates. Nevertheless, since a steel containing 0.1% or more C produces large amount of cementites and then does not give favorable magnetic characteristics even when the ferritic grains are refined, the C content is necessary to limit to 0.1% or less.
As an example of the high tensile strength hot-rolled steel sheet containing 0.1% or less C and having both fine ferritic grains and fine precipitates, xe2x80x9cCurrent Advances in Materials and Processesxe2x80x9d published by The Iron and Steel Institute of Japan, Vol.5, p.1863 (1992), (Literature No.1), proposes a precipitation strengthened hot-rolled steel sheet having excellent stretch flangeability owing to the addition of Ti. The ductility of the hot-rolled steel sheet is improved by reducing the content of C so as to decrease amount of cementites, while the high tensile strength thereof is attained by adding large amount of Ti. JP-B-8-26433, (the term xe2x80x9cJP-Bxe2x80x9d referred to herein signifies xe2x80x9cexamined Japanese Patent Publicationxe2x80x9d), discloses a high tensile strength hot-rolled steel sheet consisting mainly of 0.03 to 0.05% C and 0.10 to 0.20% Ti, with improved workability by controlling the volume of both pearlites and low temperature transformation phase. JP-A-63-166931, (the term xe2x80x9cJP-Axe2x80x9d referred to herein signifies xe2x80x9cunexamined Japanese Patent Publicationxe2x80x9d), discloses a method for manufacturing a high tensile strength hot-rolled steel sheet having high magnetic flux density on the base of a Sixe2x80x94Mn steel containing Ti and B. According to the description of JP-A-63-166931, B is added to improve the hardenability, and a bainitic structure is formed by cooling the hot-rolled steel sheet at a cooling speed of 30xc2x0 C./sec or more. And the combination of transformation strengthening and TiC precipitation strengthening is utilized to attain both high tensile strength and high magnetic flux density. JP-A-58-91121 discloses a method for manufacturing a high tensile strength hot-rolled steel sheet having high magnetic flux density with the addition of Ti, where the content of Si is limited to 0.10% or less, and the tensile strength is increased by TiC precipitation strengthening.
However, the high tensile strength hot-rolled steel sheet according to Literature No.1 does not have high magnetic permeability because of the large amount of Ti. The high tensile strength hot-rolled steel sheet according to JP-B-8-26433 shows low magnetic permeability due to the existence of pearlites and of low temperature transformation phase. The method of JP-A-63-166931 fails to give high magnetic permeability because of the large content of Si and Mn. Particularly, the significantly large content of 0.21 to 0.30% Si and 1.22 to 1.90% Mn induces problems in surface properties and weldability. By the method of JP-A-58-91121, since the Ti addition is as large as close to Ti/C=1, the magnetic permeability degrades because of the solid solution Ti inevitably left during hot-rolling process, and the surface properties and the weldability degrade due to the large content of Si which is added for obtaining high tensile strength.
An object of the present invention is to provide a high tensile strength hot-rolled steel sheet having good workability, high magnetic permeability, high magnetic flux density in a strong magnetic field, and having strength of at least 490 MPa, and to provide a method for manufacturing thereof.
The object is attained by a high tensile strength hot-rolled steel sheet, which comprises 0.04 to 0.09% C, 0.1% or less Si, 0.5 to 1.5% Mn, 0.02% or less P, 0.01% or less S, 0.1% or less Al, 0.001 to 0.008% N, and 0.01 to 0.15% Ti, by mass %, the content of ingredient thereeach satisfies the following equation (1), and the ferritic grain size xcex1 (xcexcm) satisfies the following equation (2):
[C]+7xc3x97[Si]+0.1xc3x97[Mn]+[P]+14xc3x97[S]+1.75xc3x97[Al]
xe2x80x83+23xc3x97[N]+[Ti]+18xc3x97[O]+7xc3x97[Cu]+18xc3x97[Sn]+7xc3x97[Mo]+
1.7xc3x97[Cr]+70xc3x97[B]+7xc3x97[Ca]+14xc3x97[Zr]+14xc3x97[V]+7xc3x97[Nb]
xe2x89xa62xe2x80x83xe2x80x83(1)
3xe2x89xa6xcex1xe2x89xa660xc3x97[Ti]+8xe2x80x83xe2x80x83(2)
where, [X] denotes the content (mass %) of element X.
For the prevention of softening of heat-affected zone (HAZ) after welding, a preferred high tensile strength hot-rolled steel sheet is the one which comprises 0.04 to 0.09% C, 0.1% or less Si, 0.5 to 1.5% Mn, 0.02% or less P, 0.01% or less S, 0.1% or less Al, 0.001 to 0.008% N, 0.2% or less Mo, and 0.4% Cr, and at least one selected from 0.01 to 0.15% Ti and 0.005 to 0.05% Nb, by mass %, the content of ingredient thereeach satisfies the above equation (1), and the ferritic grain size xcex1 (xcexcm) satisfies the following equations (3) and (4):
3xe2x89xa6xcex1xe2x89xa660xc3x97([Ti]+[Nb])+8xe2x80x83xe2x80x83(3) 
[Mn]+6xc3x97[Cr]+15xc3x97[Mo]xe2x88x920.05xc3x97xcex1xe2x89xa62xe2x80x83xe2x80x83(4) 
Furthermore, instead of specifying the ferritic grain size as described above, the same effect is obtained with a high tensile strength hot-rolled steel sheet comprising 0.04 to 0.09% C, 0.1% or less Si, 0.5 to 1.5% Mn, 0.02% or less P, 0.01% or less S, 0.1% or less Al, 0.001 to 0.008% N, and 0.01 to 0.15% Ti, by mass %, or with a high tensile strength hot-rolled steel sheet comprising 0.04 to 0.09% C, 0.1% or less Si, 0.5 to 1.5% Mn, 0.02% or less P, 0.01% or less S, 0.1% or less Al, 0.001 to 0.008% N, 0.2% or less Mo, and 0.4% Cr, and at least one selected from 0.01 to 0.15% Ti and 0.005 to 0.05% Nb, by mass %, wherein the pearlite area percentage is 5% or less.
That type of high tensile strength hot-rolled steel sheet can be manufactured by a method comprising the steps of: preparing a slab of steel comprising the chemical compositions described above; hot-rolling the slab at a finishing temperature of from Ar3 transformation point to 880xc2x0 C.; cooling the hot-rolled steel sheet to a temperature range of from 630 to 680xc2x0 C. at an average cooling speed of from 15 to 35xc2x0 C./sec. Alternatively, that type of high tensile strength hot-rolled steel sheet can be manufactured by a method comprising the steps of: preparing a slab of steel comprising the chemical compositions described above; hot-rolling the slab at a finishing temperature of from Ar3 transformation point to 880xc2x0 C.; cooling the hot-rolled steel sheet to 680xc2x0 C. at an average cooling speed of 15xc2x0 C./sec or more; and coiling the cooled steel sheet at a coiling temperature of from 500 to 650xc2x0 C.